1. Field of the Invention
The invention relates to a process for the hydrothermal preparation of crystalline sodium disilicate from quartz sand and caustic soda solution and/or from aqueous solutions of amorphous sodium disilicate.
2. Statement of Related Art
The preparation of crystalline sodium salts of silicic acids with a SiO.sub.2 /Na.sub.2 O ratio of from 2:1 to 3:1 is usually effected by annealing sodium silicate glasses or by heating sodium carbonate and quartz (DE-OS 31 00 942).
Willgallis and Range (Glastechnische Berichte [Glass Technology Reports] 37 (1964), 194-200) describe the preparation of .alpha.-, .beta.- and .gamma.-Na.sub.2 Si.sub.2 O.sub.5 by annealing melted and unmelted dehydrated soda water glass. These products have a layer structure. That they comprise crystalline forms is evident from the X-ray diffraction spectra. The authors show that various crystal forms are obtained, dependent on the temperature.
Benecke and Lagaly, in Am. Mineral. 62 (1977), 763-771, describe a process for the preparation of a hydrated crystalline sodium silicate having a kanemite structure (approximately conforming to the composition of NaHSi.sub.2 O.sub.5). In said process, in a first step SiO.sub.2 is dispersed in methanol, and cold caustic soda solution is added thereto. This process is expensive because of the controlled addition of the individual substances and requires particular safety precautions because of the use of flammable methanol.
In DE-OS 34 17 649 there has been described a process wherein a hydrated sodium silicate is admixed with crystal nuclei, the reaction mixture is dehydrated by heating, and the dehydrated mixture is maintained at a temperature which is at least 450.degree. C., but is below the melting point thereof, until the crystalline sodium silicate having a layer structure has been formed.
Upon the dehydration (upon evaporation) of water glass solutions or of solutions of conventional sodium silicates (having no layer structure) with a molar ratio of SiO.sub.2 /Na.sub.2 O of from 1.9:1 to 3.5:1, amorphous products are obtained. It is only at temperatures in excess of 450.degree. C. and in the course of several hours that these products will crystallize in the presence of an aqueous phase (DE-OS 34 17 649). In general, several hours are required for this reaction. At temperatures of between 600.degree. C. and 850.degree. C., minutes, and for certain silicates even fractions of a minute, occasionally may be sufficient.
EP-A 0 320 770 describes a process for the preparation of an X-ray crystalline sodium disilicate (sodium silicate having a layer structure) by dissolution in water of an X-ray crystalline layer silicate and subsequent evaporation at an elevated temperature. This process already employs a crystalline sodium layer silicate as the starting material and has been specifically directed to the production of sodium disilicate-containing washing and cleaning agents. Said printed publication only describes a mode of processing a crystalline sodium disilicate by dissolving same in water and subsequently evaporating the water, which crystalline sodium disilicate, however, primarily has been obtained by other methods.
Likewise it has been known to one of ordinary skill in the art that the dissolution of amorphous sodium disilicate in water and subsequent evaporation again results in the formation of an amorphous sodium disilicate.
According to prior art it has not been possible so far to directly produce a crystalline sodium disilicate of a high purity by a hydrothermal route in a commercially realizable mode.
J. Franke (Bull. Soc. Chim. 950, p. 454 et seq.) describes attempts for a hydrothermal preparation of sodium disilicate. For recovering the reaction product from quartz and a sodium hydroxide solution, it was attempted in expensive apparatuses, in part by using crystallization nuclei, to induce crystallization of the reaction product. Then the obtained reaction products were allowed to crystallize for several weeks with cooling, whereby mixtures comprising amorphous and crystalline sodium disilicate were frequently obtained.